Process for separation of alkyl phenols by azeotropic distillation with a nonpolar branched alkene



Aug. 13. 1968 L. L. PARISSE 3,397,124

PROCESS FOR SEPARATION OF ALKYL PHENOLS BY AZEOTROPIC DISTILLATION WITHA NONPOLAR BRANCHED ALKENE Filed Dec. 9. 1966 POLAR ALKYL PHENOLS NONPOLAR AND NON-POLAR ALKYL PHENOLS BRANCHED ALKENE AZEOTROPICDISTILLATION AZEOTROPE OF POLAR NON-POLAR ALKYL PHENOLS AND ALKYLPHENQLS NON-POLAR BRANCHED ALKENE POLAR NON POLAR BRANCHED ALKYL PHENOLSALKENE IN VEN TOR.

LOU/5 L PA 19/555 United States Patent 3,397,124 PROCESS FOR SEPARATIONOF ALKYL PHENOLS BY AZEOTROPIC DISTILLATION WITH A NON- POLAR BRANCHEDALKENE Louis L. Parisse, Oil City, Pa., assignor to Koppers Company,Inc., a corporation of Delaware Filed Dec. 9, 1966, Ser. No. 600,492 7Claims. (Cl. 203-52) ABSTRACT OF THE DISCLOSURE Polar alkyl phenolspreferentially azeotrope with a nonpolar branched alkene so that theazeotroped alkyl phenol is separable by distillation from less polaralkyl phenols. The nonpolar branched alkene should have about 12 carbonatoms and its boiling point should be within 30 C. of the alkyl phenolwith which it is to azeotrope.

Background of the inventipn This invention relates to a process for theseparation and purification of alkyl phenols, particularly, to theseparation of alkyl phenols from a mixture of alkyl phenols having closeboiling points but differing degrees of polarity by adding to themixture an azeotroping agent which preferentially azeotropes at leastone of the, alkyl phenols.

Mono and polymethyl phenols are naturally occurring phenols found incoal tar distillates such as derived from bituminous coal. Dimethylphenols, as well as trimethyl phenols, are also obtained as by-productsin the alkylation of phenol to produce Z-methyl phenol (o-cresol). Thedimethyl phenols exist in six isomeric forms, as do also the trimethylphenols.

Certain of the methylphenol isomers are valuable as intermediates forthe synthesis of, for example, antioxidants, while others may be used aspolymerizable rnonomers. For example, 2,6-dimethyl phenol is a valuableisomer which is used in the preparation of aromatic polyothers such aspolyphenylene oxide. It is therefore desirable to obtain the isomer in ahighly purified form. Unfortunately, when 2,6-dimethyl phenol isproduced synthetically or is produced from coal tar distillates,monomethyl phenols (cresols) are present. For example, when 2,6dimethylphenol is present in what is called the oresol distillate or fraction ofcoal tar by-product-named for the monomet'hyl phenols (cresols) whichare in this fractionit is almost impossible to separate the 3-methylphenol and 4-methyl phenol from the dimethyl phenol by ordinarydistillation techniques due to the close boiling points of thecompounds. (201 C. for 2,6-dimethyl phenol, 202.8 C. -for 3 methylphenol, and 202.5 C. for 4-methyl phenol.)

Summary of the invention It has now been discovered that alkyl phenolmixtures having very close boiling points may be separated by theaddition of a branched alkene azeotroping agent to the mixture.

In accordance with the invention, alkyl phenols having close bolingpoints may be separated by:

(a) Adding to a mixture of alkyl phenols wherein at least one alkylphenol is more polar than the others, a nonpolar branched higher alkenehaving a boiling point 3,397,124 Patented Aug. 13, 1968 within about 30C. of the more polar alkyl phenol to form an azeotrope of the alkene andthe more polar alkyl phenol.

(b) Distilling the mixture to separate the azeotrope from the less polaralkyl phenol.

Detailed description While the exact mechanism by which the alkenepreferentially azeotropes with one or more of the alkyl phenols is notcompletely understood, it is believed that the alkene, being a nonpolarmolecule, is preferentially attracted to the most polar alkyl phenol orphenols, for example, to 3-methyl phenol and 4-methyl phenol rather than2,6- dimethyl phenol due to the steric hindrance and thus low polarityof the 2,6-dimethyl phenol.

The branched higher alkenes useful in this invention are those havingfrom 10-14 and preferably 12 carbon atoms. The particular alkeneselected should have a boiling point within about 30 C. of the polaralkyl phenol with which it is to form an azeotrope.

Examples of suitable branched higher alkenes are those formed bytrimerization of methylpropene (iso-butylene). These trimers boil atabout l-l77 C. and have a specific gravity at 60" F. of 0.763.Conveniently, they may be obtained as a by-product in the butylation ofcresols with isobutylene to produce butyl-cresols. If the trimer beobtained in this fashion, it may be purified to remove cresylicimpurities by caustic scrubbing followed by fractional distillation.

The alkene polar alkyl phenol azeotrope contains about -90% by weightalkene at atmospheric pressure. It is therefore necessary, when thedistillation is conducted at atmospheric pressure, to use amounts ofalkene in a weight ratio to the polar alkyl phenol with which it is toazeotrope of at least 6 to 1. However, since the weight percent of thetrimer in the azeotrope decreases with pressure, lesser amounts oftrimer can be used if the distillation is carried out at higherpressures. It is preferable to add an excess of alkene usually about 10to 1 to 20 to 1 for atmospheric distillations. The excess alkene is thenremoved in the distillation after the alkene-polar alkyl phenolazeotrope. Some of the less polar alkyl phenol may also come over inthis fraction in which case, the fraction may be recycled to increasethe overall yield.

After the azeotropic distillation the azeotroped alkyl phenol may beseparated from the alkene by conventional means such as steamdistillation or crystallization.

The accompanying flow sheet and the following examples will serve tofurther illustrate the practice of the invention.

EXAMPLE I 200 grams of a C branched alkene (trimer of isobutylene) wasadded to 200 grams of -a mixture of 2,6-dimethyl phenol and 'monomethylphenols which had been previously analyzed and found to contain 91.5%2,6-dimethyl phenol, 1.9% Z-met-hyl phenol and 6.6% 3-methyl phenol and4-methyl phenol. The mixture was pumped to a still equipped with a helixpacked distillation column. The mixture was distilled at atmosphericpressure with a 19:1 reflux ratio. The first series of cuts werecollected up to a boiling point at 176 C. A second series of cuts werethen collected between 176 and 200 C. The third series 3 of cuts werecollected at 201 C. Analysis of the fractions by vapor phasechromatography revealed the following constituents one alkyl phenol thatis more polar than the other in the mixture comprising:

(a) adding to said mixture a nonpolar branched higher TABLE I AreaPercent Composition Fraction Boiling Weight Weight 2,6 3-methyl Point,0. (grams) (percent) 012 dimethyl 2-methyl phenol,

Trimer phenol phenol 4-methyl phenol 40. 7 10.2 79.0 0.3 38. 8 9. 7 79.6 1. 6 39. 8 9. 9 86.3 6. 7 19. 4. 9 88. 3 7.4 23. 5 5. 9 92.3 7. 7 18.64. 6 90.8 9.2 38. 4 9. 6 90.7 9.3 11.6 2. 9 67.1 32. 9 9.1 2.3 4. 8 95.2 89. 4 22. 3 0. 06 99. 94 45.0 11.2 100 Residue 26.0 6. 5 0.1 99.8

Fractions 14 contained almost all of the monomethyl phenols whilefractions 10 and 11 were essentially pure 2,6-dimethyl phenol. Fractions5-9 are suitable for recycle back to the still or the alkene may beseparated from the 2,6-dimethyl phenol by steam distillation.

Similar results are obtained when the weight ratios of trimer tomonomethyl phenol impurities are varied from 6 parts by weight trimerper part by weight monomethyl phenol mixture to parts by weight trimerper part by weight of monomethyl phenol.

EXAMPLE II The pure 2,6-dimethyl phenol obtained in the third series ofcuts (fractions 10-11) in Example I was further tested for purity byobtaining the freezing point of the material. A portion of the liquidwas placed in a test tube equipped with a stirrer and a thermometer. Thetest tube was immersed in an ice bath while the contents were stirred.When definite crystallization was started, stirring was stopped and thefreezing point considered to be the maximum temperature reached aftersuper cooling. The freezing point was found to be 45.5 C.

The foregoing has presented a novel process for the separation of polaralkyl phenols from nonpolar alkyl phenols which have hitherto been verydifliculty separable. The process allows, for example, 2,6-dimethy1phenol to be separated from monomethyl phenols with very little loss of2,6-dimethyl phenol.

What is claimed is:

1. A process for the separation of at least two alkyl phenols havingclose boiling points but containing at least alkene having from 10-14carbon atoms and a boiling point within 30 C. of the said more polaralkyl phenol to form an azeotrope of the alkene and the said more polaralkyl phenol; and

(b) distilling the azeotrope from said mixture to separate themore-polar alkyl phenol from the less polar alkyl phenol.

2. The process of claim 1 wherein the nonpolar branched alkene has 12carbon atoms.

3. The process of claim 2 wherein the nonpolar branched alkene consistessentially of trimers of methylpropene.

4. The process of claim 3 wherein the mixture of alkyl phenols comprises2,6-dimethyl phenol and monomethyl phenols.

5. The process of claim 4 wherein the trimers of methylpropene have aboiling point range of l75-l77 C. and a. specific gravity at F. of0.763.

6. The process of claim 5 wherein the amount of trimer is from 6 to 20times by weight of the weight of the monomethyl phenols present in thealkyl phenol mixture.

7. The process of claim 1 wherein said morepolar alkyl phenol isseparated from said nonpolar branched higher alkene after thedistillation.

References Cited UNITED STATES PATENTS WILBUR L. BASCOMB, JR., PrimaryExaminer.

